Final answer:
The keyboard shortcut to toggle between data and field codes is Alt+F9.
Step-by-step explanation:
The keyboard shortcut Stephen can use to toggle between data and field codes is Alt+F9.
To insert a field into a building block, Stephen can press Ctrl+F9 to insert a pair of curly brackets. He can then type the field code inside the curly brackets, and finally press Alt+F9 to toggle between viewing the field code and the result of the field code. This allows him to see and edit the variable data associated with the field.